Pancake graph


In the mathematical field of graph theory, the pancake graph Pn or n-pancake graph is a graph whose vertices are the permutations of n symbols from 1 to n and its edges are given between permutations transitive by prefix reversals.
Pancake sorting is the colloquial term for the mathematical problem of sorting a disordered stack of pancakes in order of size when a spatula can be inserted at any point in the stack and used to flip all pancakes above it. A pancake number is the minimum number of flips required for a given number of pancakes. Obtaining the pancake number is equivalent to the problem of obtaining the diameter of the pancake graph.
The pancake graph of dimension n, Pn, is a regular graph with vertices. Its degree is n − 1, hence, according to the handshaking lemma, it has edges. Pn can be constructed recursively from n copies of Pn−1, by assigning a different element from the set as a suffix to each copy.

Results

Pn is super-connected and hyper-connected.
Their girth is
The γ genus of Pn is:

Chromatic properties

There are some known graph coloring properties of pancake graphs.
A Pn pancake graph has total chromatic number, chromatic index.
There are effective algorithms for the proper -coloring and total n-coloring of pancake graphs.
For the chromatic number the following limits are known:
If, then
if, then
if, then
The following table discusses specific chromatic number values for some small n.
345678910111213141516
233444?4?4?4?4?4?4?4?4?

Cycle enumeration

In a Pn pancake graph there is always at least one cycle of length ℓ, when . It implies that the graph is Hamiltonian and any two vertices can be joined by a Hamiltonian path.
About the 6-cycles of the Pn pancake graph: every vertex belongs to exactly one 6-cycle. The graph contains independent 6-cycles.
About the 7-cycles of the Pn pancake graph: every vertex belongs to 7-cyles. The graph contains different 7-cycles.
About the 8-cycles of the Pn pancake graph: the graph contains different 8-cycles; a maximal set of independent 8-cycles contains of those.

Diameter

The pancake sorting problem and obtaining the diameter of the pancake graph are equivalents. One of the main difficulties in solving this problem is the complicated cycle structure of the pancake graph.
1234567891011121314151617
01345789101113141516171819

The pancake number, which is the minimum number of flips required to sort any stack of pancakes has been shown to lie between and but the exact value remains an open problem.
In 1979, Bill Gates and Christos Papadimitriou gave an upper bound of. This was improved, thirty years later, to by a team of researchers at the University of Texas at Dallas, led by Founders Professor Hal Sudborough.
In 2011, Laurent Bulteau, Guillaume Fertin, and Irena Rusu proved that the problem of finding the shortest sequence of flips for a given stack of pancakes is NP-hard, thereby answering a question that had been open for over three decades.

Burnt pancake graph

In a variation called the burnt pancake problem, the bottom of each pancake in the pile is burnt, and the sort must be completed with the burnt side of every pancake down. It is a signed permutation, and if a pancake i is "burnt side up" a negative element i` is put in place of i in the permutation. The burnt pancake graph is the graph representation of this problem.
A burnt pancake graph is regular, its order is, its degree is.
For its variant David S. Cohen and Manuel Blum proved in 1995, that .
123456789101112
14681012141517181921

The girth of a burnt pancake graph is:

Other classes of pancake graphs

Both in the original pancake sorting problem and the burnt pancake problem, prefix reversal was the operation connecting two permutations. If we allow non-prefixed reversals then four classes of pancake graphs can be defined. Every pancake graph embeds in all higher-order pancake graphs of the same family.
NameNotationExplanationOrderDegreeGirth
unsigned prefix reversal graphThe original pancake sorting problem
unsigned reversal graphThe original problem with two spatulas
signed prefix reversal graphThe burnt pancake problem
signed reversal graphThe burnt pancake problem with two spatulas

Applications

Since pancake graphs have many interesting properties such as symmetric and recursive structures, sublogarithmic degree and diameter, and are relatively sparse, much attention is paid to them as a model of interconnection networks for parallel computers. When we regard the pancake graphs as the model of the interconnection networks, the diameter of the graph is a measure that represents the delay of communication.
Pancake flipping has biological applications as well, in the field of genetic examinations. In one kind of large-scale mutations, a large segment of the genome gets reversed, which is analogous to the burnt pancake problem.